Commit message: btrfs: fix NULL pointer dereference when deleting device by invalid id

[BUG]
It's easy to trigger NULL pointer dereference, just by removing a
non-existing device id:

 # mkfs.btrfs -f -m single -d single /dev/test/scratch1 \
				     /dev/test/scratch2
 # mount /dev/test/scratch1 /mnt/btrfs
 # btrfs device remove 3 /mnt/btrfs

Then we have the following kernel NULL pointer dereference:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: 0000 [#1] PREEMPT SMP NOPTI
 CPU: 9 PID: 649 Comm: btrfs Not tainted 5.14.0-rc3-custom+ #35
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
 RIP: 0010:btrfs_rm_device+0x4de/0x6b0 [btrfs]
  btrfs_ioctl+0x18bb/0x3190 [btrfs]
  ? lock_is_held_type+0xa5/0x120
  ? find_held_lock.constprop.0+0x2b/0x80
  ? do_user_addr_fault+0x201/0x6a0
  ? lock_release+0xd2/0x2d0
  ? __x64_sys_ioctl+0x83/0xb0
  __x64_sys_ioctl+0x83/0xb0
  do_syscall_64+0x3b/0x90
  entry_SYSCALL_64_after_hwframe+0x44/0xae

[CAUSE]
Commit a27a94c2b0c7 ("btrfs: Make btrfs_find_device_by_devspec return
btrfs_device directly") moves the "missing" device path check into
btrfs_rm_device().

But btrfs_rm_device() itself can have case where it only receives
@devid, with NULL as @device_path.

In that case, calling strcmp() on NULL will trigger the NULL pointer
dereference.

Before that commit, we handle the "missing" case inside
btrfs_find_device_by_devspec(), which will not check @device_path at all
if @devid is provided, thus no way to trigger the bug.

[FIX]
Before calling strcmp(), also make sure @device_path is not NULL.

Fixes: a27a94c2b0c7 ("btrfs: Make btrfs_find_device_by_devspec return btrfs_device directly")
CC: stable@vger.kernel.org # 5.4+
Reported-by: butt3rflyh4ck <butterflyhuangxx@gmail.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
CVE: CVE-2021-3739
CWE: CWE-703
Vulnerability description: A NULL pointer dereference flaw was found in the btrfs_rm_device function in fs/btrfs/volumes.c in the Linux Kernel, where triggering the bug requires ‘CAP_SYS_ADMIN’. This flaw allows a local attacker to crash the system or leak kernel internal information. The highest threat from this vulnerability is to system availability.

Project: linux
File: volumes.c

int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
		    u64 devid)
{
	struct btrfs_device *device;
	struct btrfs_fs_devices *cur_devices;
	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
	u64 num_devices;
	int ret = 0;

	mutex_lock(&uuid_mutex);

	num_devices = btrfs_num_devices(fs_info);

	ret = btrfs_check_raid_min_devices(fs_info, num_devices - 1);
	if (ret)
		goto out;

	device = btrfs_find_device_by_devspec(fs_info, devid, device_path);

	if (IS_ERR(device)) {
		if (PTR_ERR(device) == -ENOENT &&
		    strcmp(device_path, "missing") == 0)
			ret = BTRFS_ERROR_DEV_MISSING_NOT_FOUND;
		else
			ret = PTR_ERR(device);
		goto out;
	}

	if (btrfs_pinned_by_swapfile(fs_info, device)) {
		btrfs_warn_in_rcu(fs_info,
		  "cannot remove device %s (devid %llu) due to active swapfile",
				  rcu_str_deref(device->name), device->devid);
		ret = -ETXTBSY;
		goto out;
	}

	if (test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state)) {
		ret = BTRFS_ERROR_DEV_TGT_REPLACE;
		goto out;
	}

	if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state) &&
	    fs_info->fs_devices->rw_devices == 1) {
		ret = BTRFS_ERROR_DEV_ONLY_WRITABLE;
		goto out;
	}

	if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
		mutex_lock(&fs_info->chunk_mutex);
		list_del_init(&device->dev_alloc_list);
		device->fs_devices->rw_devices--;
		mutex_unlock(&fs_info->chunk_mutex);
	}

	mutex_unlock(&uuid_mutex);
	ret = btrfs_shrink_device(device, 0);
	if (!ret)
		btrfs_reada_remove_dev(device);
	mutex_lock(&uuid_mutex);
	if (ret)
		goto error_undo;

	/*
	 * TODO: the superblock still includes this device in its num_devices
	 * counter although write_all_supers() is not locked out. This
	 * could give a filesystem state which requires a degraded mount.
	 */
	ret = btrfs_rm_dev_item(device);
	if (ret)
		goto error_undo;

	clear_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state);
	btrfs_scrub_cancel_dev(device);

	/*
	 * the device list mutex makes sure that we don't change
	 * the device list while someone else is writing out all
	 * the device supers. Whoever is writing all supers, should
	 * lock the device list mutex before getting the number of
	 * devices in the super block (super_copy). Conversely,
	 * whoever updates the number of devices in the super block
	 * (super_copy) should hold the device list mutex.
	 */

	/*
	 * In normal cases the cur_devices == fs_devices. But in case
	 * of deleting a seed device, the cur_devices should point to
	 * its own fs_devices listed under the fs_devices->seed.
	 */
	cur_devices = device->fs_devices;
	mutex_lock(&fs_devices->device_list_mutex);
	list_del_rcu(&device->dev_list);

	cur_devices->num_devices--;
	cur_devices->total_devices--;
	/* Update total_devices of the parent fs_devices if it's seed */
	if (cur_devices != fs_devices)
		fs_devices->total_devices--;

	if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
		cur_devices->missing_devices--;

	btrfs_assign_next_active_device(device, NULL);

	if (device->bdev) {
		cur_devices->open_devices--;
		/* remove sysfs entry */
		btrfs_sysfs_remove_device(device);
	}

	num_devices = btrfs_super_num_devices(fs_info->super_copy) - 1;
	btrfs_set_super_num_devices(fs_info->super_copy, num_devices);
	mutex_unlock(&fs_devices->device_list_mutex);

	/*
	 * at this point, the device is zero sized and detached from
	 * the devices list.  All that's left is to zero out the old
	 * supers and free the device.
	 */
	if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state))
		btrfs_scratch_superblocks(fs_info, device->bdev,
					  device->name->str);

	btrfs_close_bdev(device);
	synchronize_rcu();
	btrfs_free_device(device);

	if (cur_devices->open_devices == 0) {
		list_del_init(&cur_devices->seed_list);
		close_fs_devices(cur_devices);
		free_fs_devices(cur_devices);
	}

out:
	mutex_unlock(&uuid_mutex);
	return ret;

error_undo:
	btrfs_reada_undo_remove_dev(device);
	if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
		mutex_lock(&fs_info->chunk_mutex);
		list_add(&device->dev_alloc_list,
			 &fs_devices->alloc_list);
		device->fs_devices->rw_devices++;
		mutex_unlock(&fs_info->chunk_mutex);
	}
	goto out;
}